Content addressable memory having column redundancy

ABSTRACT

A content addressable memory (CAM) having a main array including a plurality of columns of CAM cells, a spare column of CAM cells selectable to functionally replace a defective column of CAM cells in the main array, a steering circuit for steering data corresponding to the defective column to the spare column, and a global mask circuit for masking the defective column during a compare operation.

BACKGROUND

1. Field of Invention

This invention relates generally to column redundancy in contentaddressable memory devices.

2. Description of Related Art

Column redundancy has been used to improve the yield of contentaddressable memory (CAM) devices. In U.S. Pat. No. 5,319,589, Yamagataet al disclose a column redundancy technique for a dynamic CAM devicethat allows a defective column of CAM cells to be replaced with aredundant column of CAM cells. Yamagata's CAM device includes aplurality of normal bit line circuits coupled to bit line pairs ofcorresponding columns of CAM cells, a redundancy bit line circuitcoupled to a redundant bit line pair of the redundant column of CAMcells, and a switch circuit that selectively couples input/output (I/O)line pairs to the normal and redundant bit line circuits. Each column ofCAM cells includes a pair of fuses that are blown to isolate defectivecolumns from the CAM array. Similarly, the redundant column includes apair of fuses that are blown to enable the redundant column to replace adefective column.

Yamagata et al's CAM device requires two fuses per column to selectivelyenable or disable the column. The pair of fuses required for each columnundesirably increases the size of the CAM array. Further, because fusestypically do not scale well with semiconductor process technologies, theincrease in array size resulting from including fuses on each column isnot readily remedied by migration to a smaller geometry processtechnology. Thus, it would be desirable to provide a column redundancyscheme for CAM devices that is more area efficient and that scalesbetter with new process technologies.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention are illustrated byway of example and are by no means intended to limit the scope of thepresent invention to the particular embodiments shown, and in which:

FIG. 1 is a block diagram of one embodiment of a CAM device including amain array and one or more spare columns;

FIG. 2 is a block diagram of one embodiment of the CAM array of FIG. 1;

FIG. 3 is a circuit diagram of one embodiment of a compare circuit forthe CAM cells of the array of FIG. 2;

FIG. 4 is a circuit diagram of another embodiment of a compare circuitfor the CAM cells of the array of FIG. 2;

FIG. 5 is a block diagram of one embodiment of a global mask circuit inaccordance with the present invention;

FIG. 6 is a block diagram of another embodiment of a global mask circuitin accordance with the present invention;

FIG. 7 is a block diagram of another embodiment of a CAM deviceincluding a main array and at least one spare column;

FIG. 8 is a block diagram of one embodiment of a steering circuit inaccordance with the present invention; and

FIGS. 9A and 9B are block diagrams of an alternate embodiment of thesteering circuit.

Like reference numerals refer to corresponding parts throughout thedrawing figures.

DETAILED DESCRIPTION

A method and apparatus for performing column redundancy in a CAM deviceis disclosed. In the following description, for purposes of explanation,specific nomenclature is set forth to provide a thorough understandingof the present invention. However, it will be apparent to one skilled inthe art that these specific details may not be required to practice thepresent invention. In other instances, well-known circuits and devicesare shown in block diagram form to avoid obscuring the present inventionunnecessarily. Additionally, the interconnection between circuitelements or blocks may be shown as buses or as single signal lines. Eachof the buses may alternatively be a single signal line, and each of thesingle signal lines may alternatively be buses. Additionally, logicstates of various signals described herein are exemplary and thereforemay be reversed or otherwise modified as generally known in the art.

The embodiments for performing column redundancy disclosed hereinobviate the need for using fuses on each column in a CAM array. Thus,the CAM array can be designed and fabricated without having toaccommodate for numerous fuses. This allows for an efficient use ofsilicon area that scales well with smaller geometry processtechnologies.

FIG. 1 shows one embodiment of a CAM device 100 in accordance with thepresent invention. CAM device 100 includes CAM array 102 that has a mainCAM array 104 and one or more spare CAM columns 106. The spare CAMcolumns 106 are independent columns of CAM cells that may be enabled toreplace columns of CAM cells in main CAM array 104 that have one or moredefective CAM cells. The CAM cells in main CAM array 104 and sparecolumns 106 may be any type of CAM cells, including binary or ternaryCAM cells.

One or more instructions and related control signals may be provided toCAM device 100 from an instruction decoder (not shown for simplicity) tocontrol read, write, and compare operations for CAM device 100. Otherwell-known signals which may be provided to CAM device 100, such asenable signals, reset signals, and clock signals, are not shown forsimplicity.

Each row of CAM cells in array 102 is coupled to an address decoder 108via a corresponding word line WL, and to a priority encoder 110 and tomatch logic 112 via a corresponding match line ML. For one embodiment,address decoder 108 receives addresses from an address bus ABUS. Forother embodiments, address decoder 108 receives addresses from anotherbus. The word lines and match lines are represented collectively in FIG.1 for simplicity. The match lines ML provide match results for compareoperations to priority encoder 110, which determines the matching entrythat has the highest priority number associated with it and generatesthe index or address of this highest priority match. Match logic 112 maygenerate a match flag to indicate a match condition, and may generate amultiple match flag to indicate multiple matches.

Further, although not shown in FIG. 1 for simplicity, each row of CAMcells in CAM array 102 may have one or more validity bits to indicatewhether the corresponding row (or segment thereof) of CAM cells is in avalid state or an empty state. Match logic 112 may monitor the state ofthe validity bits and assert a full flag when all of the rows of CAMcells in CAM array 102 are filled with valid entries. The validity bitsmay also be provided to priority encoder 110 to generate the next freeaddress (NFA) that is available in CAM array 102 for storing new data.

Each column of CAM cells in main array 104 and spare columns 106 iscoupled to a read/write circuit 114 and to a global mask circuit 116.Global mask circuit 116 stores one or more mask patterns that maskentries in CAM array 102 for compare operations with a comparand word(e.g., search key) provided, for example, by a comparand register 118.Comparand words may be provided to comparand register 118 from acomparand bus CBUS via a steering circuit 120. For other embodiments,steering circuit 120 may provide comparand data directly to global maskcircuit 116. Read/write circuit 114 includes well-known write driversand sense amplifiers, and is coupled to a data bus DBUS via steeringcircuit 122.

Steering circuits 120 and 122 may be any well-known switching logic suchas, for example, a crossbar logic circuit, a switch matrix, translationlogic, data filter, or mapping circuit that steers data to and fromselected columns of CAM array 102 in response to one or more controlsignals provided, for example, by column address decoder 126. For otherembodiments, steering circuits 120 and 122 are the same circuit, andCBUS and DBUS are the same bus as illustrated, for example, in FIG. 7.In the embodiment of FIG. 7, the input/output (I/O) lines of steeringcircuit 702 are multiplexed between read/write circuit 114 and comparandregister 118 in a well-known manner.

Note that steering circuit 120 and/or 122 can also be used to steer readdata, write data, and comparand data bits or groups of bits to any bitlocation (or removed altogether) for other than column redundancy.

Spare columns 106 may be programmed to replace defective columns of CAMcells in main CAM array 104 as follows. Initially, main CAM array 104 istested using any generally known testing methodology and hardware todetermine if any of the CAM cells in main CAM array 104 are defective.If a defective CAM cell is detected, the column address of the defectiveCAM cell is programmed into a memory element 124 using any suitabletechnique. Memory element 124 may be any suitable storage device. Forone embodiment, memory element 124 includes a plurality of fuses thatmay be programmed with the defective column address or addresses usingwell-known laser techniques. For another embodiment, memory element 124is a flash memory device. For other embodiments, memory element 124 maybe eliminated, and the defective column address may be stored in asuitable external memory device and provided to CAM device 100 duringinitialization or reset.

When programmed, memory element 124 outputs the defective column address(DCA) to column address decoder 126, which may be any type of addressdecoder. Column address decoder 126 decodes DCA and provides the decodedcolumn address as a plurality of column control (CC) signals to globalmask circuit 116, to steering circuits 120 and 122, and to read/writecircuit 114, where each CC signal indicates whether a correspondingcolumn in main CAM array 104 is defective. In alternative embodiments,DCA may be stored in memory 124 fully decoded and column decoder 126eliminated. In response to the CC signals, steering circuits 120 and 122steer data access for the defective column of CAM cells in main CAMarray 104 to spare column 106. Thus, for write operations, steeringcircuit 122 steers data for the defective column(s) to spare column(s)106. For read operations, steering circuit 122 steers data received fromspare column(s) 106 into the correct position of the data output ontoDBUS in place of data from the corresponding defective column in themain CAM array. For compare operations, steering circuit 120 steerscomparand data corresponding to the defective column(s) in main CAMarray 104 to spare column(s) 106 for comparison with data storedtherein. Global mask circuit 116 selectively masks the defective columnin main array 104 in response to the CC signals to prevent the defectivecolumn from effectively participating in the compare operation.

FIG. 2 shows a CAM array 200 that is one embodiment of CAM array 102 ofFIG. 1. Array 200 includes main CAM array 104 having a plurality of CAMcells 202 organized in any number of rows and columns, and includesspare column 106 having a number of redundant CAM cells 202. CAM cells202 are well-known, and may be any suitable binary, ternary, SRAM-based,DRAM-based, or non-volatile CAM cell. As noted above, one or morevalidity bits may also be included. For simplicity, only one sparecolumn 106 is shown in FIG. 2, although in other embodiments sparecolumn 106 may include any number of spare columns of CAM cells 202. Forthose embodiments having more than one spare column, each spare columnhas its own steering circuit, memory element 124, column decoder 126,and corresponding CC signals.

Each row of CAM cells 202 is coupled to a match line ML and to a wordline WL. Each word line WL is driven by address decoder 108 (see alsoFIG. 1) to select one or more rows of CAM cells 202 for writing orreading. Each match line ML provides the match results of a compareoperation to priority encoder 110. A match line ML indicates a matchcondition for the row only if all CAM cells 202 in that row match thecomparand data. In some embodiments, the match line ML is pre-chargedfor the compare operation. If any CAM cell 202 in the row does not matchthe comparand data, the CAM cell(s) 202 discharges the match line MLtoward ground potential (i.e., logic low). Conversely, if all CAM cells202 match the masked comparand data, the match line ML remains in acharged (i.e., logic high) state.

Each column of CAM cells 202 in main CAM array 104 is coupled to a bitline pair BL and {overscore (BL)} and to a comparand line pair CL and{overscore (CL)}. Spare column 106 of CAM cells 202 is coupled to aspare bit line pair SBL and {overscore (SBL)} and to a spare comparandline pair SCL and {overscore (SCL)}. The bit line pairs BL and{overscore (BL)} and spare bit line pair SBL and {overscore (SBL)} areeach coupled to read/write circuit 114. Read/write circuit 114 iswell-known, and may include write drivers or buffers to provide data tobit line pairs BL and {overscore (BL)} and to spare bit line pair SBLand {overscore (SBL)} during write operations, and may include senseamplifiers to determine the logic states of bit line pairs BL and{overscore (BL)} and spare bit line pair SBL and {overscore (SBL)}during read operations.

The comparand line pairs CL and {overscore (CL)} and the spare comparandline pair SCL and {overscore (SCL)} are each coupled to global maskcircuit 116, which in turn is coupled to comparand register 118 viacorresponding global comparand lines GCL. For other embodiments, eachglobal comparand line GCL may be a pair of complementary globalcomparand lines. Global mask circuit 116 logically combines a comparandword received from comparand register 118 via GCL with one of the maskpatterns stored in global mask circuit 116 to generate a masked searchkey, and then drives the masked search key onto comparand line pairs CLand {overscore (CL)} (and onto spare complementary line pair SCL and{overscore (SCL)} if there is a defective column in main array 104) forcomparison with data in CAM cells 202.

For alternate embodiments, other CAM array architectures may be used.For example, in some embodiments, CAM array 200 may not includecomplementary comparand lines CL and {overscore (CL)}, in which case thecomplementary bit lines BL and {overscore (BL)} may be coupled tocomparand register 118 via global mask circuit 116 and be used toperform a compare operation as is generally known in the art. Similarly,spare column 106 may not include complementary spare comparand lines SCLand {overscore (SCL)}, in which case the complementary spare bit linesSBL and {overscore (SBL)} may be coupled to comparand register 118 viaglobal mask circuit 116. Additionally, CAM array 200 is shown as aNOR-based CAM array. In alternative embodiments, other CAM arrays (e.g.,NAND-based CAM arrays) may be used.

Read, write, and compare operations are more fully described below withreference to FIGS. 1 and 2. For write operations, a data word isprovided to steering circuit 122 via DBUS. If there is not a defectivecolumn in main CAM array 104, steering circuit 122 passes the data wordto read/write circuit 114, which in turn drives the data word ontocorresponding bit line pairs BL and {overscore (BL)}. The data iswritten to a row of main CAM array 104 selected by address decoder 108in a well-known manner, for example, by asserting a word line WL inresponse to an externally provided address, the NFA generated bypriority encoder 110, or any other presented address. Because there isnot a defective column in main array 104, data is not written to any ofspare columns 106. Alternately, default data may be written to the sparecolumns.

If there is a defective column in main CAM array 104, the DCA isprogrammed into memory element 124. Column address decoder 126 decodesDCA to generate the CC signals. In response to the CC signals, steeringcircuit 122 steers data corresponding to the defective column in mainarray 104 to spare column 106 via read/write circuit 114, and passesdata corresponding to non-defective columns to main CAM array 104 viaread/write circuit 114.

For example, if the third column of main CAM array 104 contains one ormore defective CAM cells, a DCA of “000010” is programmed into memoryelement 124 and provided to column address decoder 126. In response toDCA, column decoder 126 sets the third CC signal to an asserted state(e.g., logic high), and sets the remaining CC signals to un-assertedstates (e.g., logic low). The asserted state of the third CC signalcauses steering circuit 122 to steer the third bit of the data word tospare column 106. Read/write circuit 114 drives the third data bit ontospare bit line pair SBL and {overscore (SBL)} for writing to sparecolumn 106. Steering circuit 122 passes all other data bits tocorresponding non-defective columns of main CAM array 104 via read/writecircuit 114. In this manner, the defective column of main CAM array 104is mapped to spare column 106 without using fuses on each column 104 orspare column 106.

For read operations, the data word stored in a row selected by addressdecoder 108 is provided to read/write circuit 114, which in turn passesthe data word to steering circuit 122. In response to the CC signals,steering circuit 122 re-orders the bits of the data word for output toDBUS. Thus, continuing the example above, if steering circuit 122 mapsthe third bit of the data word to spare column 106 during the writeoperation, then steering circuit 122 re-maps the data bit read fromspare column 106 to the third bit position of the data word output toDBUS.

For compare operations, a comparand word is provided to steering circuit120 via CBUS. If there is a defective column in main CAM array 104, asindicated by the defective column address programmed into memory element124, steering circuit 120 steers the bit of the comparand word thatcorresponds to the defective column in main CAM array 104 to sparecolumn 106 and passes all other bits of the comparand word tocorresponding non-defective columns in main CAM array 104. Steeringcircuit 120 steers the comparand bits to comparand register 118, whichin turn forwards the comparand word to global mask circuit 116. Globalmask circuit 116 logically combines the comparand word with a maskpattern stored therein to form a masked search key, which is then drivenonto the comparand line pairs CL and {overscore (CL)} and sparecomparand line pair SCL and {overscore (SCL)}. If there is a defectivecolumn in main CAM array 104, and the global mask circuit masks (e.g.,in response to the CC signals) the comparand for the defective column,circuit 120 forwards the comparand word to the spare column via globalmask circuit 116 and comparand register 118.

For one embodiment, the unmasked bits of the search key are driven ontocorresponding comparand line pairs CL and {overscore (CL)} and/or sparecomparand lines SCL and {overscore (SCL)} to allow for comparison withdata bits stored in corresponding columns of the CAM array, while themasked bits of the search key drive corresponding comparand line pairsCL and {overscore (CL)} and/or spare comparand line pair SCL and{overscore (SCL)} to the same predetermined logic state (e.g., logiclow) to effectively preclude participation in the compare operation. Forexample, for each mask bit indicating that a column or spare column inarray 102 is not to be masked, global mask circuit 116 drives thecomparand line pair or spare comparand line pair in response to thecorresponding bit of the search key. Conversely, for each mask bitindicating that a column in array 102 is to be masked, global maskcircuit 116 drives the corresponding comparand line pair or sparecomparand line pair to the same predetermined state (e.g., logic low) sothat CAM cells 202 coupled thereto indicates a match condition,irrespective of data stored therein.

Mask patterns stored in global mask circuit 116 may be steered intoselected bit positions of global mask circuit 116 by steering circuit120 in response to the CC signals in the manner described above forcompare operations. The mask bit that corresponds to the defectivecolumn in main CAM array 104 is set to a masking state to drive thecomparand line pair of the defective column to the same predeterminedlogic state (e.g., logic low). Thus, if there is a defective column inmain CAM array 104, global masking circuit 116 masks the defectivecolumn to prevent the defective column from effectively participating inthe compare operation. For one embodiment, a logic high mask bitindicates a masked condition and a logic low mask bit indicates anunmasked condition.

FIG. 3 shows one embodiment 300 of a compare circuit for CAM cell 202that selectively discharges match line ML in response to a comparisonbetween a data bit D stored in the CAM cell and a correspondingcomparand bit C. Compare circuit 300 includes a first discharge pathincluding NMOS transistors 301-302 coupled in series between the matchline ML and ground potential and a second discharge path including NMOStransistors 303-304 coupled in series between match line ML and groundpotential. The complement of the comparand bit {overscore (C)} iscoupled to the gate of transistor 301, the data bit D is coupled to thegate of transistor 302, the comparand bit C is coupled to the gate oftransistor 303, and the complement of the data bit {overscore (D)} iscoupled to the gate of transistor 304. If the comparand bit does notmatch the data bit, one discharge path is conductive, and match line MLdischarges to ground potential to indicate the mismatch condition.Conversely, if the comparand bit matches the data bit, or if C and{overscore (C)} are both set to logic low (e.g., in response to anasserted mask bit), both discharge paths are non-conductive, and matchline ML remains in a pre-charged state (e.g., by a pre-charge circuitcoupled to the match line) to indicate the match condition.

FIG. 4 shows another embodiment 400 of a compare circuit for CAM cell202. Compare circuit 400 includes NMOS transistors 401 and 402 coupledin series between the comparand bit C and its complement {overscore(C)}, with the gate of transistor 401 coupled to the data bit D and thegate of transistor 402 coupled to the complemented data bit {overscore(D)}. An NMOS discharge transistor 403 coupled between match line ML andground potential has a gate coupled to a node 404 betweenseries-connected transistors 401 and 402. If the comparand bit does notmatch the data bit, either transistor 401 (if C=0, D=1) or transistor402 (if C=1, D=0) pulls the gate of transistor 403 to logic high andturns on transistor 403, which in turn discharges match line ML.Conversely, if the comparand bit matches the data bit, either transistor401 (if C=1, D=1) or transistor 402 (if C=0, D=0) pulls the gate oftransistor 403 to logic low to turn off transistor 403, which in turnallows match line ML to remain in a pre-charged state. If both C and{overscore (C)} are set to logic low (e.g., in response to an assertedmask bit), transistor 403 is non-conductive and allows match line ML toremain in its pre-charged state.

FIG. 5 shows a first segment 500 of one embodiment of global maskcircuit 116 (see also FIG. 1) for controlling the comparand line pair CLand {overscore (CL)} for a corresponding column of main CAM array 104.Global mask circuit 500 includes a latch 502, a mask register 504, andcomparand driver 506. Latch 502 includes a data input coupled to columncontrol signal CC and a control terminal coupled to an enable signal EN.Latch 502 may be any type of latch including, for example, a D-typeflip-flop. Mask register 504 stores a mask bit MSK for the correspondingcolumn of main CAM array 104, and may be any suitable register or memoryelement. Comparand driver 506 includes inputs coupled to receive CC fromlatch 502, MSK from mask register 504, and a corresponding comparand bitCMP from comparand register 118, and includes outputs coupled tocomparand line pair CL and {overscore (CL)}.

During initialization or reset of CAM device 100 (or at anotherdetermined time), EN is asserted (e.g., to logic high) and causes CC tobe latched into latch 502. Comparand driver 506 logically combines CC,MSK, and CMP to selectively drive comparand line pair CL and {overscore(CL)}. If CC is asserted to indicate that the column is defective,comparand driver 506 drives comparand line pair CL and {overscore (CL)}to the same logic state (e.g., logic low) to prevent the column fromparticipating in the compare operation, regardless of MSK and CMP.Conversely, if CC is not asserted, comparand driver 506 drives CL and{overscore (CL)} in response to CMP if MSK is set to an unmasked stateor drives CL and {overscore (CL)} to the same predetermined state if MSKis asserted to a masked state. The logic function performed by comparanddriver 506 is summarized in the truth table below. It is to beunderstood that numerous logic circuits may be used to implement thelogic function illustrated in Table 1 (including alternatives thatlogically complement one or more of the signals), and therefore specificcircuit configurations of comparand driver 506 are not provided hereinso as to not obscure the invention.

TABLE 1 CC (x) MSK (x) CMP (x) CL {fraction (CL)} 0 0 0 0 1 0 0 1 1 0 01 0 0 0 0 1 1 0 0 1 0 0 0 0 1 0 1 0 0 1 1 0 0 0 1 1 1 0 0

FIG. 6 shows a second segment 600 of one embodiment of global maskcircuit 116 (see also FIG. 1) for controlling the spare comparand linepair SCL and {overscore (SCL)} coupled to a corresponding spare column106. Global mask circuit 600 includes a latch 602, a mask register 604,comparand driver 606, and a memory element 608. Latch 602, which may beany type of latch including, for example, a D-type flip-flop, includes adata input coupled to a spare column enable signal (SPEN) stored inmemory element 608 and a control terminal coupled to EN. Memory element608 may be any suitable memory device. For one embodiment, memoryelement 602 is a fuse that may be programmed either electrically or witha laser. For other embodiments, memory element 608 may be a non-volatilesemiconductor memory such as, for example, flash memory. Mask register604 stores a mask bit MSK for the spare column 106, and may be anysuitable register or memory element. Comparand driver 506 includesinputs to receive SPEN from latch 602, MSK from mask register 604, and acomparand bit CMP steered from the defective column in main CAM array104 to spare column 106 via steering circuit 120. Comparand driver 606also includes outputs coupled to spare comparand line pair SCL and{overscore (SCL)}.

The logic state of SPEN indicates whether spare column 106 is selectedto replace a defective column in main CAM array 104. During manufactureof CAM device 100, memory element 608 is initially set to an un-assertedstate (e.g., SPEN=0) that disables spare column 106. If testing revealsa defective column in main CAM array 104, memory element 608 is set toan asserted state (e.g., SPEN=1) that enables spare column 106. Duringinitialization or reset of CAM device 100 (or at another determinedtime), EN is asserted (e.g., to logic high) and causes SPEN to belatched into latch 602. Comparand driver 606 logically combines SPEN,MSK, and CMP to selectively drive spare comparand line pair SCL and{overscore (SCL)}. If SPEN is not asserted, comparand driver 606 drivesspare comparand line pair SCL and {overscore (SCL)} to the same logicstate (e.g., logic low) to prevent spare column 106 from participatingin compare operations. Conversely, if SPEN is asserted, comparand driver606 drives SCL and {overscore (SCL)} in response to CMP if MSK is set toan unmasked state or drives SCL and {overscore (SCL)} to the same stateif MSK is asserted to a masked state. The logic function of comparanddriver 606 is the same as illustrated above in table 1, where the SPENsignal of FIG. 6 corresponds to the CC signal of FIG. 5. For someembodiments, global mask circuits 500 and 600 are the same circuit.

As mentioned above, memory elements 124 (FIG. 1) and 608 (FIG. 6) may beany suitable volatile or non-volatile storage device, including flashmemory. Embodiments using flash memory for memory elements 124 and 608allow for easier migration to smaller geometry process technologies.Further, embodiments for which memory elements 124 and 608 are flashmemory allow the defective column address and SPEN, respectively, to beprogrammed into CAM device 100 at any time after fabrication. Thus, forexample, if a column in main CAM array 104 becomes defective after aperiod of use by a customer, the customer may program the defectivecolumn address into CAM device 100 and set SPEN to an asserted state tofunctionally replace a defective column in main CAM array 104 with aspare column 106. In this manner, the customer may repair CAM device 100long after CAM device 100 is packaged and sold to the customer. Forother embodiments, CAM device 100 may include standard testingarchitecture such as, for example, the well-known JTAG standard, thatallows for an in-system repair of CAM device 100. The ability to repairCAM device 100 after packaging and subsequent use may increase theuseful life of CAM device 100 and improve its reliability. In contrast,because fuses on a semiconductor die are typically blown before the dieis sorted and packaged, devices that use such fuses on the bit lines orcolumns to implement column redundancy cannot be readily repaired by thecustomer.

FIG. 8 shows a steering circuit 800 that is one embodiment of steeringcircuits 120 and 122 of FIG. 1 and/or steering circuit 702 of FIG. 7.Steering circuit 800 includes a spare column select circuit 802 and aplurality of column select circuits 808(0)-808(x) that steer data intothe CAM array in response to the CC signals. For one embodiment, sparecolumn select circuit 802 and column select circuits 808 aremultiplexers, although any suitable select logic may be used. Sparecolumn select circuit 802 is coupled to spare column 106 via a sparecolumn write circuit 804, and column select circuits 808(0)-808(x) arecoupled to corresponding columns in main CAM array 104 via column writecircuits 806(0)-806(x), respectively. Write circuits 804 and806(0)-806(x) form part of read/write circuit 114. For one embodiment,write circuits logic 804 and 806(0)-806(x) perform a logical ANDfunction. For simplicity, global mask circuit 116 and comparand register118 are not shown in FIG. 8. Further, although described below as havingonly one spare column, spare column 106 may include any number of sparecolumns.

The spare column enable signal SPEN is provided to write circuit 804 andto column decoder 126. The defective column address stored in memoryelement 124 is provided as a control signal to spare column selectcircuit 802, which includes a plurality of inputs to receive data bitsD(0)-D(x) from DBUS and includes an output terminal coupled to sparecolumn 106 via write circuit 804. The defective column address is alsoprovided to column address decoder 126 via an inverter circuit 810.Column decoder 126 decodes the logically complemented DCA to generatecolumn control signals CC(0)-CC(x), each of which indicates whether acorresponding column in main CAM array 104 is defective. For thisembodiment, a logic high state for CC indicates that the correspondingcolumn is non-defective, and a logic low state for CC indicates that thecorresponding column is defective.

Column control signals CC(0)-CC(x) are coupled to first input terminalsof write circuits 806(0)-806(x), respectively, and to control terminalsof column select circuits 808(0)-808(x), respectively. Each writecircuit 806(0)-806(x) includes a second input coupled to receive acorresponding data bit D(0)-D(x), and includes an output coupled to acorresponding column of main CAM array 104. Each select circuit808(0)-808(x) includes a first input coupled to a corresponding columnof main CAM array 104, a second input coupled to spare column 106, andan output to provide a corresponding data bit D(0)-D(x) onto DBUS.

For write operations, a data word is provided onto DBUS as data bitsD(O)-D(x) and provided to write circuits 806(0)-806(x), respectively,and to spare column select circuit 802. If there is not a defectivecolumn in main CAM array 104, SPEN is set to an un-asserted state (e.g.,to logic low), which in turn disables column decoder 126 and disablesspare column write circuit 804. When disabled, spare column writecircuit 804 prevents data from being written to spare column 106, forexample, by driving the spare bit line pair (not shown in FIG. 8 forsimplicity) to the same predetermined state, and column decoder 126drives all column control signals CC(O)-CC(x) to an un-asserted state(e.g., logic high state). The un-asserted states of CC(O)-CC(x) causecorresponding column write circuits 806(0)-806(x) to pass respectivedata bits D(0)-D(x) to corresponding columns of main CAM array 104.

If there is a defective column in main CAM array 104, the DCA isprogrammed into memory element 124, and SPEN is set to an asserted state(e.g., logic high). In response to DCA, spare column select circuit 802passes the data bit D corresponding to the defective column in main CAMarray 104 to spare column 106 via write circuit 804, which is enabled bythe asserted state of SPEN. The selected data bit may be written tospare column 106 by driving the spare bit line pair to a state indicatedby the selected data bit, as described above with respect to FIGS. 1 and2. The asserted state of SPEN also enables column decoder 126, which inturn decodes the complemented DCA to assert one of column controlsignals CC(0)-CC(x). The asserted CC signal causes corresponding writecircuit 806 to prevent data from being written to the defective columnof main CAM array 104, for example, by driving the defective column'sbit line pair to the same predetermined state, while the un-asserted CCsignals allow write circuit 806 to pass the corresponding data bits tonon-defective columns of main CAM array 104.

For compare operations, steering circuit 800 operates in a similarmanner. A comparand word is provided to steering circuit 800 on DBUS asbits D(0)-D(x) (note that DBUS in this example may be the CBUS of FIG.1). Spare column select circuit 802 routes the comparand bitcorresponding to the defective column of main CAM array 104 to sparecolumn 106 via the comparand register and global mask circuit (not shownfor simplicity) in response to DCA, and column select circuits808(0)-808(x) pass the other comparand bits to correspondingnon-defective columns of main CAM array 104 in response to theun-asserted CC signals. If there is not a defective column in main CAMarray 104, the un-asserted state of SPEN disables column decoder 126,which in turn provides un-asserted CC signals to corresponding globalmask cicruits.

For read operations, data bits stored in columns of the addressed row ofmain CAM array 104 are provided to first inputs of corresponding columnselect circuits 808(0)-808(x), and the data bit stored in spare column106 is provided to second inputs of column select circuits808(0)-808(x). Column select circuits 808(0)-808(x) route either thedata bit read out from the corresponding column of main CAM array 104 orthe data bit read out from spare column 106 in response to CC(0)-CC(x),respectively. Thus, for each non-defective column, CC is set to anun-asserted state, which causes the corresponding column select circuit808 to route data read from the non-defective column onto DBUS.Conversely, for each defective column, CC is set to an asserted state,which causes the corresponding column select circuit 808 to route dataread from the spare column 106 onto DBUS.

For alternate embodiments, steering circuit 122 may, during writeoperations, shift the defective and all subsequent (e.g., lesssignificant or right-most) bits of the data word by one bit whensteering the data word to read/write circuit 114. For example, if thethird column of main CAM array 104 is defective, steering circuit 122may steer the third data bit to the fourth column of main CAM array 104,steer the fourth data bit to the fifth column of main CAM array 104, andso on, and steer the last data bit to spare column 106. In suchalternate embodiments, during read operations, steering circuit 122re-shifts the same bits by one bit position to the left when outputtingthe data word to DBUS. Further, in these alternate embodiments, steeringcircuit 120 steers comparand data to CAM array 102 by shifting thedefective bit and all less-significant bits by one bit position to theright.

FIG. 9A shows an exemplary circuit configuration 900 for the alternateembodiment of steering circuits 120 and 122 described above that steersdata access for a defective column during write and compare operations.Steering circuit 900 includes a plurality of multiplexers (MUXes)902(1)-902(x), each of which includes inputs to receive a correspondingdata bit and a previous data bit, a control terminal to receive acorresponding CC signal, and an output coupled to a corresponding columnof the CAM array. Thus, the first (e.g., most significant or left-most)data bit D[0] is provided to the first column (e.g., column 0) of mainCAM array 104 and to a first input of MUX 902(1), which includes asecond input to receive the second data bit D[1], a control terminal toreceive CC(1), and an output coupled to the second column (e.g., column1) of main CAM array 104. The second data bit D[1] is also provided to afirst input of MUX 902(2), which includes a second input to receive thethird data bit D[2], a control terminal to receive CC(2), and an outputcoupled to the third column (e.g., column 2) of main CAM array 104. Thelast (e.g., least significant or right-most) data bit D[x] is providedto a second input of MUX 902(x) and to the spare column 106. MUX 902(x)includes a first input to receive the second-to-last data bit D[x−1], acontrol terminal to receive CC(x), and an output coupled to the lastcolumn (e.g., column x) of main CAM array 104.

If there is not a defective column in the main CAM array, the CC signalsare un-asserted (e.g., to logic low), and each MUX 902 passes thecorresponding data bit from its second input to the correspondingcolumn. For example, MUX 902(1) passes D[1] to column 1 in response toan un-asserted CC(1), MUX 902(2) passes D[2] to column 2 in response toan un-asserted CC(2), and so on, where MUX 902(x) passes D[x] to columnx in response to an un-asserted CC(x).

If there is a defective column in the main CAM array, steering circuit900 shifts the defective and all subsequent bits of the data word by onebit when steering the data word to the CAM array. For example, if column1 is defective, CC(1) and all subsequent CC signals are asserted (e.g.,to logic high). Thus, MUX 902(2) steers D[1] to column 2 in response tothe asserted CC(2), and so on, where MUX 902(x) steers D[x−1] to columnx in response to the asserted CC(x). The spare column is enabled toreceive D[x]. Note that for steering circuit 900, the logic state ofCC(1), which corresponds to the defective column 1, is a don't carebecause data steered to the defective column 1 via MUX 902(1) will noteffectively participate in compare operations.

FIG. 9B shows an exemplary circuit configuration 910 for the alternateembodiment of steering circuits 120 and 122 described above that steersdata access for a defective column during read operations. Steeringcircuit 910 includes a plurality of MUXes 904(0)-904(x), each of whichincludes inputs to coupled to corresponding adjacent columns of CAMarray 102, a control terminal to receive a corresponding CC signal, andan output to provide a corresponding data bit. Thus, MUX 904(0) includesinputs coupled to the first and second columns (e.g., columns 0 and 1)of the main CAM array, a control terminal to receive CC(0), and anoutput to provide the first (e.g., most significant or left-most) databit D[0]. MUX 904(1) includes inputs coupled to the second and thirdcolumns (e.g., columns 1 and 2) of the main CAM array, a controlterminal to receive CC(1), and an output to provide the second data bitD[1]. MUX 904(x) includes inputs coupled to the last column (e.g.,column x) of the main CAM array and to the spare column, a controlterminal to receive CC(x), and an output to provide the last (e.g.,least significant or right-most) data bit D[x].

If there is not a defective column in the main CAM array, the CC signalsare un-asserted (e.g., to logic low), and each MUX 904 outputs data fromthe corresponding column. For example, MUX 904(0) outputs data fromcolumn 0 as D[0] in response to an un-asserted CC(0), MUX 904(1) outputsdata from column 1 as D[1] in response to an un-asserted CC(1), and soon, where MUX 904(1) outputs data from column x as D[x] in response toan un-asserted CC(x).

If there is a defective column in the main CAM array, steering circuit910 shifts the defective and all subsequent bits of the data word by onebit when reading from the CAM array. For example, if column 1 isdefective, CC(0) is un-asserted, and CC(1) and all subsequent CC signalsare asserted (e.g., to logic high). Thus, MUX 904(0) steers data fromcolumn 0 as D[0] in response to the un-asserted CC(0), MUX 904(1) steersdata from column 2 as D[1] in response to the asserted CC(1), and so on,where MUX 904(x) steers data from the spare column as D[x] in responseto the asserted CC(x).

The alternate embodiment described above with respect to FIG. 9A doesnot use a select circuit having inputs to receive all the data bitsD[0:x] such as, for example, the select circuit 802 of FIG. 8, andtherefore advantageously simplifies signal routing within the steeringcircuit.

For another embodiment, multiple spare columns may be included. For thisexample, each MUX 902 in FIG. 9A will receive the input of two previousbits as well as the default bit for its corresponding bit storage in themain CAM array. Additionally, each MUX will receive a second CC signalcorresponding to the second spare column. Similarly, MUX 904 in FIG. 9Bwill receive the input of two subsequent bits as well as the default bitfor its corresponding bit storage in the main CAM array.

It is to be noted that for all present embodiments, read and writeoperations to the defective column need not be disabled because theglobal mask circuit prevents the defective column from effectivelyparticipating in compare operations (e.g., by masking the defectivecolumn). Further, column redundancy for CAM devices discussed herein maybe combined with CAM row redundancy techniques to further improve yield.

What is claimed is:
 1. A content addressable memory (CAM), comprising: a main array including a plurality of columns of CAM cells; a spare column of CAM cells selectable to functionally replace a defective column of CAM cells in the main array; a steering circuit for steering data corresponding to the defective column to the spare column; and a global mask circuit for masking the defective column during a compare operation.
 2. The CAM device of claim 1, wherein the global mask circuit masks the defective column in response to a defective column address.
 3. The CAM device of claim 1, further comprising: a column decoder for generating a plurality of column control signals in response to a defective column address, each column control signal indicating whether a corresponding column of CAM cells is defective.
 4. The CAM device of claim 1, wherein each column of CAM cells in the main array is coupled to a bit line pair and a compared line pair and the spare column is coupled to a spare bit line pair and a spare comparand line pair.
 5. The CAM device of claim 4, wherein the global mask circuit forces the comparand line pair of the defective column to the same predetermined state to prevent participation in the compare operation.
 6. The CAM device of claim 5, wherein the global mask circuit includes a plurality of first segments, each corresponding to a column of CAM cells in the main array and comprising: means for driving the comparand line pair of the column in response to a mask bit, a comparand bit, and a column control signal indicating whether the column is defective.
 7. The CAM device of claim 6, wherein the means for driving the comparand line pair comprises: a latch for latching the column control signal in response to an enable signal; a register for storing the mask bit; and logic for selectively masking the comparand bit in response to the mask bit and the column control signal.
 8. The CAM device of claim 6, wherein the global mask circuit includes a second segment corresponding to the spare column of CAM cells, comprising: means for driving the spare comparand line pair in response to a mask bit, a comparand bit, and a spare column enable signal indicating the existence of the defective column.
 9. The CAM device of claim 8, wherein the means for driving the spare comparand line pair masks the spare comparand bit if the spare column enable signal indicates there is not a defective column in the main array.
 10. The CAM device of claim 8, wherein the means for driving the spare comparand line pair comprises: a latch for latching the spare column enable signal in response to a control signal; a register for storing the mask bit; and logic for selectively masking the comparand bit in response to the mask bit and the spare column enable signal.
 11. The CAM device of claim 3, wherein the steering circuit comprises: a plurality of write circuits corresponding to the plurality of columns of the main array, each write circuit having a first input to receive a data bit for the corresponding column, a second input to receive the corresponding column control signal, and an output coupled to the corresponding column; a plurality of first select circuits corresponding to the plurality of columns of the main array, each select circuit having a fist input coupled to the corresponding column, a second input coupled to the spare column, a control terminal to receive the column control signal for the corresponding column, and an output to provide a corresponding data bit; and a second select circuit having inputs to receive data bits corresponding to each column in the main array, a control terminal to receive the defective column address, and an output coupled to the spare column.
 12. The CAM device of claim 11, wherein the write circuits comprise AND gates.
 13. The CAM device of claim 11, wherein the first and second select circuits comprise multiplexers.
 14. The CAM device of claim 1, wherein the spare column comprises a plurality of spare columns.
 15. A content addressable memory (CAM) device, comprising: a main array including a plurality of columns of CAM cells; a spare column of CAM cells; and means for disabling a defective column of CAM cells in the main array without including fuses for the defective column.
 16. The CAM device of claim 15, further comprising: means for enabling the spare column of CAM cells to functionally replace the defective column of CAM cells without including fuses for the spare column.
 17. The CAM device of claim 16, wherein each column of CAM cells in the man array is coupled to a bit line pair and a comparand line pair and the spare column is coupled to a spare bit line pair and a spare comparand line pair.
 18. The CAM device of claim 17, wherein the means for disabling forces the comparand line pair to the same predetermined state if the corresponding column is defective.
 19. The CAM device of claim 17, wherein the means for disabling and the means for enabling comprise a global mask circuit that stores one or more mask patterns for compare operations in the CAM device.
 20. The CAM device of claim 19, wherein the global mask circuit includes a plurality of first segments, each corresponding to a column of CAM cells in the main array and comprising: means for driving the comparand line pair of the column in response to a mask bit, a comparand bit, and a column control signal indicating whether the column is defective.
 21. The CAM device of claim 20, wherein the means for driving the comparand line pair masks the comparand bit if the column control signal indicates the corresponding column is defective.
 22. The CAM device of claim 20, wherein the means for driving the comparand line pair comprises: a latch for latching the column control signal in response to an enable signal; a register for storing the mask bit; and logic for selectively masking the comparand bit in response to the mask bit and the column control signal.
 23. The CAM device of claim 20, wherein the global mask circuit includes a second segment corresponding to the spare column of CAM cells, comprising: means for driving the spare comparand line pair in response to a mask bit, a comparand bit, and a spare column enable signal indicating the existence of the defective column.
 24. The CAM device of claim 23, wherein the means for driving the spare comparand line pair masks the spare comparand bit if the spare column enable signal indicates there is not a defective column in the main array.
 25. The CAM device of claim 23, wherein the means for driving the spare comparand line pair comprises: a latch for latching the spare column enable signal in response to a control signal; a register for storing the mask bit; and logic for selectively masking die comparand bit in response to the mask bit and the spare column enable signal.
 26. A method for functionally replacing a defective column of content addressable memory (CAM) cells of a CAM array with a spare column of CAM cells, comprising: steering data corresponding to the defective column of CAM cells to the spare column of CAM; and masking the defective column during a compare operation.
 27. The method of claim 26, further comprising: programming a column address indicative of the defective column into a memory element.
 28. The method of claim 27, further comprising: decoding the column address to generate a plurality of column control signals.
 29. The method of claim 26, wherein the masking comprises driving a comparand line pair of the defective column to the same predetermined state to prevent the defective column from participating in the compare operation.
 30. The method of claim 29, wherein the masking further comprises: for each column in the main array, logically combining a comparand bit, a mask bit, and a corresponding column control signal to selectively drive the comparand line pair.
 31. A method for functionally replacing a defective column of content addressable memory (CAM) cells in a CAM array with a spare column of CAM cells, comprising: steering data corresponding to the defective column of CAM cells to a next column of CAM cells; steering data corresponding to each column subsequent to the defective column to an adjacent column of CAM cells; and steering data from a last column of CAM cells to the spare column of CAM cells.
 32. The method of claim 31, further comprising: programming a column address indicative of the defective column into a memory element.
 33. The method of claim 32, further comprising: decoding the column address to generate a plurality of column control signals.
 34. The method of claim 33, further comprising: masking the defective column during a compare operation in response to the column control signals.
 35. The method of claim 34, wherein the masking comprises driving a comparand line pair of the defective column to the same predetermined state to prevent the defective column from participating in the compare operation.
 36. The method of claim 35, wherein the masking further comprises: for each column in the main array, logically combining a comparand bit, a mask bit, and the corresponding column control signal to selectively drive die comparand line pair. 